The present invention relates to a terahertz transmitter or receiver module More specifically, the present invention relates to a robust modularly packaged terahertz transmitter and receiver module.
The present invention is concerned with the generation of terahertz electromagnetic radiation by a pulsed laser in a commercially packaged system In previous applications such as in a lab environment, a laser can be pointed directly through space at an optical switching element with negligible dispersive effects. To allow the commercial use of such a system the present invention must be industrially hardened and packaged. A laser pulse in a room environment may be deflected by objects or people and will suffer degradation from atmospheric effects, unacceptable conditions in an industrial environment. The laser must also be realigned constantly due to environmental effects on the material properties of the alignment mechanisms. By incorporating optical fiber cable and rugged packaging in the present invention, the laser light is given a predetermined path of travel and allows the present invention to be precisely aligned, ruggedly seated, and bundled into compact form. A ruggedly packaged, fiber-delivered, terahertz system allows people unfamiliar with the setup, alignment, or adjustment of ultrafast lasers, semiconductor physics, and optics, to use a time-domain terahertz system for experiments and applications outside the lab environment.
Another advantage of the fiber-delivered terahertz system is the ease with which the system can be reconfigured for use in either transmission or reflection experiments. Presently, this type of reconfiguration takes days. With the system of the present invention reconfiguration takes minutes. The terahertz transceivers, in particular, need to be built using advanced telecommunication packaging techniques in order to build these units with sufficient precision and maintain their ruggedness to such that they may be used in an industrial environment By directing short ( less than 1-ps) pulses of light to the substrate by using a fiber-delivery system, we allow for the terahertz transceivers to be freely positioned As discussed previously, present time-domain terahertz and frequency-domain terahertz systems are usable only in the research laboratory By using fiber optic packaging techniques, we are able to make these devices manufacturable and usable by people outside the research community. The basic concept compnses anchoring a fiber near the terahertz transmitter and/or receiver, giving the present invention a substantial advantage over previous free-space systems.
However, there are some drawbacks to simply butting the fiber up to the terahertz transmitter or receiver device First, the generated terahertz radiation couples into the high dielectric substrate material preferentially over air, thus improving the efficiency of the emitter if the fiber is butted up to the substrate, radiation would be coupled into the fiber, away from the emission aperture, reducing efficiency Also, the size of the beam of light emitting from the end of a single-mode fiber is about 5 xcexcm or larger. This is too large to adequately generate or detect the terahertz radiation.
Another difficulty of present terahertz systems is the difficulty in aligning the optical axis (comprised of the optical fiber and the terahertz element) and the terahertz optics (comprised of the terahertz element and the attached hemispherical lens) The hemisphencal optic is either aplanatic or collimating as disclosed in U.S. Pat. No. 5,789,750, expressly incorporated by reference herein. It should be noted that this lens can be made from any number of materials that are effective at this wavelength regime Some examples are high-resistivity silicon ( greater than 1 kxcexa9-cm), alumina, sapphire, or even polyethylene Furthermore, this lens can be anti-reflection coated to enhance terahertz output using a number of materials including parylene.
The new and improved system of the present invention solves these and other problems found in the prior art as will be illustrated and discussed hereinafter.
The present invention provides an intermediate or relay optic (GRIN or other focusing element coupled to the optical pulse delivery fiber) that allows for an adjustable optical spot size, which enhances the sensitivity of the overall system by improving the efficiency of the terahertz transmitter and the receiver. This spot of light must be aligned to the terahertz transmitter or receiver device with sub-micron precision. By using the relay optic we obtain a lever arm on this alignment, effectively increasing the accuracy by a factor proportion to the magnification of the relay optic. That is, the lens transforms movement of the optical fiber into a smaller movement of the focused optical spot.
The alignment problem found in the prior art is solved by the present invention, for example, by using mounting plates made of a maternal similar to the lens material. The terahertz element is mounted onto a window mounting plate using alignment marks (or fiducials) that are micro-fabricated onto the plate, and the relay optic and optical fiber are mounted to an optic mounting plate. Once assembled, both subassemblies can be aligned independent of the other. By carefully designing these various elements the entire system becomes much more manufacturable and rugged than previously obtainable. The use of the mounting plates also makes it easier to environmentally seal or hermetically seal the terahertz transmitter or receiver package. The mounting plates could be made of alumina a material compatible with such a process, while the lens could be made of any other material, and with any other optical design, that would be appropriate for the application at hand.
Moreover the present invention includes the use of a fiber to deliver short optical pulses to a terahertz transmitter or receiver More specifically, the invention uses a fiber, along with an intermediate optic, to deliver a focused beam of short ( less than 1-ps) optical pulses to a terahertz device. This device is the element containing the active area or volume in which the delivered light power either (1) interacts to produce out-going terahertz electromagnetic radiation, or (2) responds with in-coming terahertz radiation to produce an electrical signal or alter the optical beam in a measurable manner. In the first case, the device is a transmitter, and in the second, it is a receiver This terahertz device can be either a photoconductive element such as that disclosed in U.S. Pat. Nos. 5.729,017, 5,420,595 and 5,663,639 expressly incorporated by reference herein, or an electroptic or magneto-optic device such as those disclosed in U.S. Pat. Nos. 5,952,815 or 6,111,416 expressly incorporated by reference herein.
Further objects and advantages of the present invention will become apparent by reference to the following description of the preferred embodiment and appended drawings wherein like reference numbers reflect the same feature, element or component